1. Field of the Invention
The present invention relates to an image forming device for forming images on a recording medium using a developing agent.
2. Description of the Related Art
A laser printer is an example of a well-known image forming device for printing images on a recording medium. In such a laser printer, a laser device radiates a laser beam, based on image data, onto a photosensitive member to form electrostatic latent images thereon. Selectively transferring toner, which is a developing agent in powdered form, onto the photosensitive member develops the electrostatic latent images into visible toner images, which are then transferred onto a paper sheet or other recording medium.
Laser printers capable of forming color images are also well known. Such laser printers store different colors of toner, such as cyan (C), magenta (M), yellow (Y), and black (BK) toner. Electrostatic latent images are formed one after the other on the photosensitive member for each color, and then developed into visible toner images using toner in the corresponding colors. The toner images are transferred one after the other in a primary transfer operation onto an intermediate transfer member so that the different colored toner images overlap one on the other. As a result, a multicolor toner image is formed on the intermediate transfer member by the overlapping monochrome images. Afterward, the multicolor toner image is transferred onto the recording medium in a secondary transfer operation, to form a color image on the recording medium.
However, not all of the toner that forms the multicolor toner image is transferred from the intermediate transfer member to the recording medium. Therefore, a cleaning unit is normally provided to clean the intermediate transfer member by removing the residual toner that clings to the intermediate transfer member after the secondary transfer.
The cleaning unit can be switched between a non-cleaning mode, wherein the cleaning unit cannot remove toner from the intermediate transfer member, and a cleaning mode, wherein the cleaning unit can remove toner from the intermediate transfer member. By switching the mode of the cleaning unit at appropriate timings, residual toner alone can be selectively removed from the intermediate transfer member without damaging the multicolor toner image. Normally, the cleaning unit is in contact with the intermediate transfer member in the cleaning mode and separated from the intermediate transfer member in the non-cleaning mode.
When the cleaning unit is switched between these modes, vibration is generated in the laser printer and also the load on the rotating intermediate transfer member can fluctuate. These can warp or distort the image being output.
For example, when the cleaning unit is brought into or out of contact with the intermediate transfer member during the primary transfer, the intermediate transfer member vibrates due to the action of contact or separation. Further, the fluctuation in the load on the rotating intermediate transfer member produces temporary fluctuations in rotation speed. Therefore, the toner image that is in the process of being transferred in the primary transfer process can be distorted because of the mode switching operation of the cleaning unit. Accordingly, the corresponding portion of the image outputted after the secondary transfer will be distorted. There is also a problem particular to color laser printers because the different colored toner images are stacked one on top of the other on the intermediate transfer member. That is, the overlap between different colored toner images can be shifted when the rotational speed of the intermediate transfer member changes, resulting in distortion in the colors of the output image.
Japanese Patent-Application Publication (Kokai) No. HEI-10-48967 discloses an image forming device that does not perform the mode-switching operation of the cleaning unit during the primary transfer operation. Accordingly, distortion in the output image that results from generation of vibration in the image forming device and fluctuations of the rotation load during primary transfer that can be caused by contact and separation of the cleaning unit can be suppressed to a certain extend.
However, in the image forming devices such as laser printers that form images by forming an electrostatic latent image and performing a primary transfer and a secondary transfer, it is not possible to sufficiently suppress distortion in the output image by merely controlling the contact and separation operations of the cleaning unit by taking the primary transfer into consideration.
It is an objective of the present invention to provide an image forming device capable of suppressing distortion and shifts in outputted images due to the contact and separation operations of a cleaning unit.
In order to achieve the above and other objects, according to the present invention, there is provided an image forming device including an endless photosensitive member, an exposure unit, a developing unit, an endless image bearing member, a secondary transfer unit, a cleaning unit, and a control unit. The endless photosensitive member moves in a first direction. The exposure unit performs exposure operations for exposing the photosensitive member at an exposure position to form a latent image on the photosensitive member. The developing unit develops the latent image into a developing-agent image on the photosensitive member at a developing position that is downstream from the exposure position in the first direction. The endless image bearing member contacts the photosensitive member at a primary transfer position that is downstream from the developing position in the first direction. The image bearing member moves in a second direction. The developing-agent image is transferred from the photosensitive member onto the image bearing member at the primary transfer position in primary transfer operations. The secondary transfer unit performs secondary transfer operations for transferring the developing-agent image from the image bearing member onto a recording medium at a secondary transfer position that is downstream from the primary transfer position in the second direction. The cleaning unit is switched between a contact condition where the cleaning unit is in contact with the image bearing member at a cleaning position and a separation condition where the cleaning unit is separated from the image bearing member. The cleaning position is downstream from the secondary transfer position and upstream from the primary transfer position in the second direction. The cleaning unit in the contact condition removes residual developing agent from the image bearing member after the secondary transfer operations. The control unit switches the cleaning unit between the contact condition and the separation condition during a stopped period wherein no latent image is being formed during the exposure operations.